An explanation for why water disappeared from the surface of Mars shortly after its formation, but remained present on Earth, producing conditions suitable for the evolution of life, is published in this week’s Nature. The study suggests that basalts on Mars can hold more water than those on Earth, and that some of the water on Mars may be buried underground.

Surface water has existed on Earth for most of geological time, but Mars lost its surface water very early on. Previous research has suggested that Mars lost much of it to space after the collapse of its magnetic field, but this does not account for all of the missing water.

Jon Wade and colleagues calculated the relative volumes of water that could be removed from each planet’s surface through reacting with lava to form basaltic crust. They found that the relatively iron-rich basalts on Mars can hold about 25 per cent more water than those on Earth, and that these basalts transport this water to the Martian interior (mantle). The authors conclude that early in Earth’s geological history, Earth had a more buoyant crust and a steeper geothermal gradient (the rate at which temperature increases with increasing depth) than did Mars. Earth’s water was thus prevented from being buried in the upper mantle and remained close to the surface.